According to Osho, Aum was chosen to signify the seventh plane because it is beyond meaning: composed only of the three primal sounds a‑u‑m, which by themselves carry no concept. To prevent conceptual capture, it’s rendered as a pictorial symbol of three parts. Heard as an inner hum in thoughtless stillness, it functions as a doorway-question mark pointing to the source beyond language.
Aum is a meaning-free sound-picture made from basic sounds that helps stop thinking and guides you to the silent source of everything.